Conventional GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a mechanically latched circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of differential current when a line-to-ground fault occurs.
GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical receptacle configuration, as shown, for example, in U.S. Pat. No. 4,568,997, to Bienwald et al, includes test and reset pushbuttons and a lamp or light-emitting diode (LED) which indicates that the circuit is operating normally. When a ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens both sides of the AC line. The tripping of the circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. In order to reset the GFCI device, the reset button is depressed in order to close and latch the circuit breaker, and this also causes the LED to illuminate once again.
Portable GFCI devices have been designed for use in situations where the available AC power supply circuit does not include a central or receptacle-type GFCI device. These portable devices may be incorporated into line cords, extension cords or plug-in units, and are often used with power tools and other types of potentially hazardous power equipment at construction sites and the like. Examples of portable GFCI devices may be found in U.S. Pat. No. 4,197,567, to Dietz et al, and in U.S. Pat. No. 5,229,730, to Legatti et al. However, like the receptacle-type GFCI devices described previously, portable GFCI devices typically rely on mechanical circuit breakers to trip in response to the ground fault condition. Mechanical circuit breakers add undesirable complexity and expense to the GFCI circuit, and are also subject to failure due to the mechanical nature of the tripping and latching functions.
Newer types of GFCI devices employ relays, rather than circuit breakers or other types of mechanical latching devices, to interrupt the load power when a ground fault condition occurs. Art electronic circuit controls the flow of current to the relay coil, and the relay contacts serve to open and close both sides of the AC line in response to the presence or absence of a ground fault condition. In these devices, only a simple momentary pushbutton switch is needed to perform the reset function, since latching of the relay contacts is performed electronically rather than mechanically. This results in a simpler, less expensive and more reliable device. As disclosed in the aforementioned copending patent application of Thomas M. McDonald, Ser. No. 08/115,020, a relay-type GFCI device can be designed to incorporate a manual set feature, wherein the reset pushbutton must be depressed before power can be applied to an AC load. This provides protection against unexpected starting of the AC load when the GFCI device is initially connected or after a power supply interruption, which can be dangerous when power equipment is involved.
Unfortunately, the substitution of a relay-type circuit for a mechanical circuit breaker is difficult in the case of portable GFCI devices, particularly those which are incorporated into AC line cord plugs. These devices must be relatively small in order to perform their intended function, and a conventional relay, which must be mounted on a circuit board along with the relatively large number of electrical components required for the latching and tripping functions, takes up a considerable amount of space. Ideally, it would be desirable to reduce the size of a relay-type GFCI device so that it can be incorporated into an AC line cord plug of relatively small dimensions.